High throughput screening method, array assembly and system

ABSTRACT

A liquid chemical stripping or cleaning solution is selected by combinatorial high throughput screening. A high throughput screening well array assembly includes (A) a metal substrate and (B) a mask that defines an array of wells on the substrate. A combinatorial high throughput screening system includes (A) a metal substrate and (B) a mask that defines an array of wells on the substrate and a reaction vessel to receive the well array assembly.

This application is a division of application Ser. No. 09/666,381, filedAug. 20, 2000, now U.S. Pat. No. 6,420,178, issued Jul. 16, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to a high throughput screening (HTS)method, array assembly and system. Particularly, it relates to a method,assembly and system to identify a chemical stripping or cleaningsolution.

A typical gas turbine engine includes a compressor, a combustor and aturbine. Gases flow axially through the engine. Compressed gasesemerging from the compressor are mixed with fuel and burned in thecombustor. Hot products of combustion emerge from the combustor at highpressure and enter the turbine where thrust is produced to propel theengine and to drive the turbine, which in turn drives the compressor.

The compressor and the turbine include alternating rows of rotating andstationary coated airfoils. High temperature combustion gases degradethe coatings through either hot corrosion or oxidation. Gases thatcirculate through the airfoils, particularly during operation on theground, also include particles of sand, dust, oxides of calcium,magnesium, aluminum, silicon and mixtures that have been ingested by theengine. The oxides can combine to form particularly deleteriouscalcium-magnesium-aluminum-silicon-oxide systems (Ca—Mg—Al—Si—O),referred to as CMAS. These contaminants can be in a molten state and caninfiltrate pores and openings in engine parts that can lead to crackformation and part failure. Other contaminants may include iron andnickel oxides, sodium vanadates, sodium sulfates, sodium phosphates andthe like.

Consequently, gas turbine components such as an airfoil must beperiodically repaired by removing degraded coatings, mechanicallyrepairing the airfoil and recoating the airfoil surface. Removal of thedegraded coating can be accomplished through one or more chemicalstripping or cleaning immersions. Repair of turbine engine parts canalso involve cleaning cracks, crevices and surfaces to completely removeCMAS and other oxides, organic and inorganic impurities and dirt priorto alloy filling and brazing. A typical repair process consistssequentially of a dirt clean, coating strip, then a fluoride ioncleaning (FIC) or etching prior to weld/braze repair.

Current cleaning/stripping solutions are not as effective and selectiveas desired. Also, new stripping solutions must often be developed whennew base metal airfoils are developed or when the airfoils are providedwith new coatings. Typically, “one-at-a-time” experiments are used toidentify a new solution. In these experiments, scrapped engine-runairfoil pieces are placed in a beaker of solution and immersed in a hotwater bath. The solution can be evaluated for stripping or cleaningeffectiveness first by visual inspection and then by cross-sectionalmicroscopy of cut and polished pieces. This process is time consuming.Sometimes, several months of work is involved to screen a few dozensolutions at most and then to optimize one promising solution.Part-to-part and intrapart coating variability can complicate theevaluation process. The stripping of a solution on a different cut partpiece can be difficult to determine from sample piece to sample piece.This can result in elimination of a promising solution too early in thescreening process. There is a need for a method to rapidly andefficiently screen large numbers of chemical solutions for stripping orcleaning of an airfoil.

BRIEF SUMMARY OF THE INVENTION

The invention incorporates a combinatorial chemistry approach toscreening and optimizing solution mixtures for chemical stripping orcleaning of a gas turbine component coating. The method comprisesselecting a gas turbine component chemical stripping or cleaningsolution by combinatorial high throughput screening (CHTS).

In another embodiment, the invention relates to a method, comprisingassembling a mask onto a test substrate to define a well array on thetest substrate, establishing a combinatorial library of candidate liquidreactants by depositing a candidate liquid reactant into each well ofthe array in contact with a region of the substrate, effecting reactionof each candidate liquid reactant with the substrate and evaluating eachregion of the substrate to select a best reactant from among thecandidate liquid reactants.

In another embodiment, the invention relates to a high throughputscreening well array assembly. The assembly comprises (A) a metalsubstrate and (B) a mask that defines an array of wells on thesubstrate.

In still another embodiment, the invention relates to a combinatorialhigh throughput screening system. The system includes a well arrayassembly comprising (A) a metal test substrate and (B) a mask thatdefines an array of wells on the substrate and a reaction vessel toreceive the well array assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded schematic representation of a multilayer wellassembly;

FIG. 2 is a schematic representation of a well design;

FIG. 3 is a top view of a cleaning/stripping pattern;

FIG. 2 is a schematic representation of a single well design;

FIG. 3 is a top view of a cleaning/stripping pattern under a singlewell;

FIGS. 4, 5 and 6 are top views of a metal coupon substrate after threesequential experiments using the well design of FIGS. 1, 2 and 3;

FIG. 7 is a graph of molarity change as a function of NiAI coatingremoval and contact area (mask radius) for a constant well volume of 3.5ml;

FIG. 8 is a schematic representation of a method and system to select astripping solution;

FIG. 9 is a schematic representation of another method to select astripping solution;

FIG. 10 is a schematic representation of an embodiment of the multilayerwell assembly;

FIG. 11 is a schematic representation of another embodiment of themulti-layer well assembly; and

FIG. 12 is a table showing a 96 member stripping solution experimentalspace and results.

DETAILED DESCRIPTION OF THE INVENTION

In experimental reaction systems, each potential combination ofreactant, catalyst and condition must be evaluated in a manner thatprovides correlation to performance in a production scale reactorCombinatorial organic synthesis (COS) is a high throughput screening(HTS) methodology that was developed for pharmaceuticals. COS usessystematic and repetitive synthesis to produce diverse molecularentities formed from sets of chemical “building blocks.” As withtraditional research, COS relies on experimental synthesis methodology.However instead of synthesizing a single compound, COS exploitsautomation and miniaturization to produce large libraries of compoundsthrough successive stages, each of which produces a chemicalmodification of an existing molecule of a preceding stage. The librariescomprise compounds that can be screened for various activities.

The technique used to prepare such libraries involves a stepwise orsequential coupling of building blocks to form the compounds ofinterest. For example, Pirrung et al., U.S. Pat. No. 5,143,854 disclosesa technique for generating arrays of peptides and other molecules using,for example, light-directed, spatially-addressable synthesis techniques.Pirrung et al. synthesizes polypeptide arrays on a substrate byattaching photoremovable groups to the surface, of the substrate,exposing selected regions of the substrate to light to activate thoseregions, attaching an amino acid monomer with a photoremovable group tothe activated region, and repeating the steps of activation andattachment until polypeptides of desired lengths and sequences aresynthesized.

Typically, a combinatorial high throughput screening method (CHTS) ischaracterized by parallel reactions at a micro scale. In one aspect,CHTS can be described as a method comprising (A) an iteration of stepsof (i) selecting a set of reactants; (ii) reacting the set and (iii)evaluating a set of products of the reacting step and (B) repeating theiteration of steps (i), (ii) and (iii) wherein a successive set ofreactants selected for a step (i) is chosen as a result of an evaluatingstep (iii) of a preceding iteration.

The present invention relates to a combinatorial chemistry approach toscreening and optimizing solution mixtures for chemical cleaning andstripping of airfoil coatings. The CHTS can comprise (A) steps of (i)selecting a candidate stripping or cleaning solution; (ii) effectingstripping or cleaning of a metal substrate with the solution under aselected reaction condition; and (iii) evaluating a product of thestripping or cleaning step; and (B) reiterating (A) wherein a successivesolution or condition selected for a step (i) or step (II) is selectedas a result of an evaluating step (iii) of a preceding iteration of astep (A)

The CHTS can comprise steps of preparing a metal test substrate,assembling a mask that defines a well array onto the substrate,depositing a candidate airfoil chemical stripping or cleaning solutioninto a well of the array in contact with a region of the substrate toeffect stripping or cleaning of the region and evaluating a product ofthe stripping or cleaning.

These and other features will become apparent from the drawings andfollowing detailed discussion, which by way of example withoutlimitation describe preferred embodiments of the present invention.

FIG. 1 is a schematic representation of a multilayer well array assembly10 and FIG. 2 is a schematic representation of a single well from 10. InFIG. 1, a flat metal coupon base 14 is provided upon which a coatingand/or dirt is deposited. The base 14 is furnace cycled to reproduceengine-run conditions on a specimen coupon or the base is a selectedcoupon section of an engine-run gas turbine component. As shown in FIG.1, the multilayer well array assembly 10 includes the base 14, a contactwell mask 16, reservoir array plate 18 and lid 20. Contact well mask 16can be fabricated from rubber, plastic, teflon® material, photoresist,or other suitable material. FIG. 1 shows contact well mask 16 with anarray of contact wells 24 and the reservoir array plate 18 with an arrayof reservoir wells 22. The reservoir well 22 contains a bulk of thestripping or cleaning solution. The contact wells 24 are shown withuniform cross-sections. However, the contact wells 24 can representdifferent cross-sections. The multilayer well array assembly 10 canaccommodate replaceable contact well masks 16 with different shaped orpositioned contact wells 24 to form various shaped or positioned contactareas 26 as shown in FIG. 3. In addition, the well mask 16 can beflexible to accommodate different contact areas or slight curvatures orsurface roughnesses of the metal coupon base 14.

Finally, lid 20 is provided to prevent solution evaporation andspillage. Lid 20 can have pinholes or the like located above each wellto avoid pressure build-up from evolved gases. To enhance the sealing ofthe wells to substrate, hydrophobic agents such as wax or silicone RTV,can be applied to seal a reservoir 18 and contact well mask 16 to a base14. Additionally, lid 20 and contact well mask 16 can each be providedwith a raised lip for sealing with a complementary structure. The base14, well mask 16, array plate 18 and lid 20 are fitted together as shownin FIG. 1 to form the composite multilayer well array assembly 10.

FIG. 2 shows a contact well that has a high well 22volume-to-contact-surface-area ratio. In FIG. 2, a resevoir well 22 isshown atop a contact well 24. A ratio of reservoir well 22 volume andcontact well 24 area required to avoid solution depletion can beestimated from a coating composition of the metal base and thickness tobe removed. For example, 5 moles of HCI are theoretically required todissolve a mole of a NiAI coating according to the following tworeactions.

Ni(s)+2H⁺⇄Ni²⁺(aq)+H₂(g)

Al(s)+3H⁺⇄Al³⁺(aq)+H₂(g)

A reservoir well 24 size can be estimated by molarity change of HCI as afunction of coating removed (in microns of thickness). FIG. 7 showsmolarity change as a function of NiAI coating removal and contact area(well radius). Percent solution depletion is reduced for large solutionconcentrations, large well volumes, thin coatings and small contactareas. For example, a 3M HCI solution becomes a 2.6M solution after 25microns of coating are removed using a contact well radius equal toreservoir well radius (0.75 cm). Shrinking the contact well radius to0.25 cm (while maintaining a 3.5 ml reservoir well volume) results inalmost no change in the solution molarity.

As described above, the multilayer well array assembly 10 can include acontact well mask 16 that permits varying a location of the contact well24 within the area of a larger well reservoir well 22. FIG. 3 is a topview of a cleaning/stripping pattern 26 that can be imposed by usingthree separate contact well masks 16. Additionally, a contact mask 16may be provided that is replaceable. FIGS. 4, 5 and 6 are schematicrepresentations of stripping patterns after different contact well masksare used to isolate contact of different stripping or cleaning solutionswith metal coupon base 14. In this embodiment, a mask 16 that defines afirst contact area for a first iteration of a selection method can bereplaced by a second mask that defines a second contact area for a nextiteration of the selection method. For example, FIG. 4 illustrates astripping or cleaning pattern 30 on a metal coupon base 14 that isdetermined by a contact area of a first mask 16 in a first iteration ofthe method. Then the first mask is replaced by a second mask for asecond iteration of the method. The second mask provides a contact areathat is different from the contact area of the first mask. FIG. 5illustrates a stripping or cleaning pattern 32 on metal coupon base 14after the second iteration of the method. The FIG. 5 shows a side byside stripping pattern from stripping of the first and second iteration.Then the second mask is replaced by a third mask for a third iterationof the method to provide the stripping or cleaning pattern 34 shown inFIG. 6.

In this manner, a single metal coupon base 14 can be used inreiterations of the method of the invention to select a suitable gasturbine component chemical stripping or cleaning solution. For example,a resevoir array plate 18 with large reservoir wells 22 in combinationwith 3 different contact well plates; can be used to define 72 (3×24)different cleaning and stripping experiments on a single coupon base 14.In one embodiment, different solutions can be used in the reservoirwells 22. In another embodiment, the same solution is used in a specificreservoir during successive experiments with a different contact wellmask and varying time or temperature.

FIG. 8 is a schematic representation of a system 40 of the invention toselect a stripping solution. The system 40 includes furnace 42 tosimulate engine-run conditions on a test coupon, array assembly 10(shown without lid 20), X-Y positioning stage 44, solution dispensingsystem 46, controller 48, agitator/heater 58 and evaluator 50. Thedispensing system 46 includes pipettes 52 and valves 54 used inconjunction with an array of wells 12 of multilayer well array assembly10 (without lid 20) and solution dispensing containers 56. X-Ypositioning stage 44 positions the array of wells 12 beneath a line ofpipettes 52 for delivery of test solutions from reagent containers 56.

With reference to FIG. 8 and FIG. 9, a method 60 for selecting astripping or cleaning solution can comprise preparing 62 a coated testcoupon base 14 by applying a coating to a substrate. Examples ofsubstrates include NiAl, PtAl, MCrAlY, yttrium-stabilized zirconia,chromides, etc. Examples of substrates (or base metals) include Ni-basedsuperalloys in both equiaxed and single crystal form, such as Rene N5,GTD111, etc, and Co-based alloys such as FSX414. The coated coupon istreated 64 to simulate engine run conditions. Treating step 64 can be afurnace annealing, furnace cycling (i.e., repeated heating and cooling)or a burner rig test, which involves cyclic exposure to hot combustiongas impingement. Generally the treating step 64 is carried out in anapparatus such as a furnace generally designated 42 in FIG. 8.

The well array assembly is then assembled 66 except for lid 20. Thencandidate stripping or cleaning solutions are loaded 68 into respectivewells 12 of the array assembly 10. The solutions can be automaticallydispensed by means of any suitable dispenser such as an inkjet.Preferably, the solutions are dispensed by the arrangement of FIG. 8. Inoperation, successive lines of wells 12 are positioned beneath line ofpipettes 52 by positioning stage 44, which is controlled by controller48. Controller 48 can be a computer, microprocessor or the like.Controller 48 also controls valves 54 and pipette selection of solutionfrom containers 56. The controller actuates stage 44 to position a lineof wells 12 beneath the pipettes 52. A combination of solutions isselected from containers 56 according to the controller 48 and isdelivered to each pipette 52. When the line of pipettes 52 is loaded,the controller actuates valves 54 for delivery of test solution torespective wells 12. The controller 48 records the composition of eachsolution and its position within the array of wells 12. The controller48 then actuates stage 44 to position a next line of wells 12 beneaththe pipettes 52. The method is repeated until each well 12 is loadedwith test solution. The array assembly 10 is then covered by lid 20 andsealed.

The solutions in the sealed array assembly 10 are agitated and heated 70by means of agitator/heater generally designated 58. Agitator/heater 58can be can be an automatic rocker placed in an oven, ultrasonicallyagitated hot water bath or IR lamp in combination with a rocker thatagitates and maintains the temperature of the array assembly 10 andsolution for a set period of time. For example, agitating/heating 70 canbe continued for a period of about 5 minutes to greater than 24 hours,desirably about 30 minutes to 8 hours and preferably about 30 minutes to4 hours at a temperature of about 25° C. to 200° C. desirably about 25°to 100° C. and preferably about 50° C. to 80° C. After agitating andheating 70, the array assembly 10 can be disassembled and rinsed 72.Rinsing step 72 preferably comprises repeated hot water immersions orspray water rinses. In an embodiment, step 72 can utilize a short timecaustic rinse to neutralize residual acids followed by a hot waterrinse. Short time acid immersions can also be used to remove tenaciousdirt prior to rinsing.

The extent and effectiveness of stripping or cleaning can then beevaluated 74 by analyzer 50. Analyzer 50 can be a device to conduct anelemental analysis such as an energy dispersive spectroscopy apparatus,a cross-sectional metallography device or the like. Other examples ofanalyzer 50 comprise a charge-coupled device or analyzer (CCD) camerathat detects photon wavelengths and fluxes The CCD camera can be used todetermine cleaning and stripping effectiveness.

Or analyzer 50 can be a profilometer to measure etch depth. Essentially,a profilometer measures surface roughness or profile. It provides athree dimensional topographical map of surface that permitsdetermination of amount of coating removed by a given solution. Forexample, a Dektak® (Sloan Technology Corporation, 602 E. MontecitoStreet Santa Barbara, Calif. 93103) profilometer comprises sharp needlesthat are scanned across a surface in an X-Y raster pattern to measurevertical displacement or height. Atomic force and scanning tunnelingmicroscopes (AFM's and STM's) are more refined suitable profilometersthat measure heights of single atoms. An optical profilometer (PhaseShift Technologies, Inc.) uses light interference (constructive anddestructive) to measure vertical displacement. An optical profilometerhas a resolution between that of a Dektak® device and an STM.

Another suitable analyzer is an Eagle II Microfluorescence System (EDAX,Inc.), which uses X-rays to generate characteristic wavelengthfluorescence that permits elemental identification to distinguishbetween coating and base metal. Another suitable analyzer 50 is based on“heat tint,” which involves oxidizing an entire coupon at severalhundred degrees Celsius for an hour or two and observing a color changeof the coating (or base metal). The color change identifies the amountof remaining coating or indicates whether the base metal has beencompletely exposed.

In an embodiment of the method of the invention, an agitated and heatedarray of solutions from step 70 can be subjected to an evaluating step76 to determine amount of removed coating or pH or metal concentrationof the used solution. The analyzed array is then disassembled and rinsed72 and analyzed according to step 74 to provide additional data tocontroller 48.

The steps of assembling 66, loading 68, agitating/heating 70,disassembling/rinsing and detecting/evaluating 74 can be reiterated toprovide complete test results on an experimental space. For example, themethod can be conducted with three iterations using three differentcontact well masks 16 to provide a test coupon base 14 according to FIG.6.

FIGS. 10 and 11 are schematic representations of embodiments of the wellassembly 10 of the invention. FIG. 10 shows the well assembly 10including lid 20, resevoir array plate 18 and contact well mask 16. Inthe embodiment shown, the contact well mask 16 is mounted directly ontoa surface 80 of a turbine engine part 82. In this embodiment, thesurface 80 functions as the substrate for testing In FIG. 11, lid 20,resevoir array plate 18, contact well mask 16 and the metal coupon base14 of the well assembly 10 are compressed together for processing bywell clamp 84. Well clamp 84 consists of two opposing plates 86 and 88secured together by bolts 90 and 92 to securely clamp together the wellassembly 10 for testing after loading.

The method, array assembly and system of the invention utilize CHTS torapidly screen and optimize solutions and processing conditions forstripping and cleaning of airfoil coatings. The invention providessignificant increase in experimental speed for testing these solutions.The invention can be practiced with a single coupon base. The inventioncan provide a quick screening procedure for new coatings duringdevelopment or after introduction into an engine. The invention can beused for simultaneous screening of non-destructive evaluation techniqueson an array of partially to fully stripped/cleaned coatings. In oneembodiment, the invention can be used directly on an airfoil surfaceinstead of on a coupon specimen.

The array assembly of the invention can be configured as a standard 24or 96-well plate or in a customized configuration as required by theapplication. The array assembly can be used in a variety of mixtureexperiments, such as in a single component acid diluted with waterexperiment, a mixtures of two or more acids experiment or an experimentwith one or more acids with additives such as surfactants for increasedsurface wetting and/or inhibitors to protect the underlying base metalonce the coating is removed.

These and other features will become apparent from the drawings andfollowing detailed discussion, which by way of example withoutlimitation describe a preferred embodiment of the present invention.

EXAMPLE

The invention is used in a CHTS process to select a best strippingsolution from a combination of two acids, phosphoric acid (Phos) andhydrofluorosilicic acid (HFS), Plurafac® surfactant (a polyoxyalkylenecondensate), and a Rodine® acid inhibitor. The base metal is a PtAlcoated N5 (a nickel superalloy) coupon that has been heat treated for 47hours near 2050° C. to simulate engine-run conditions.

A 96-Well multilayer well array assembly is used that has a Teflon® lid,plate and mask. The mask is a porous Teflon® material so that the arrayassembly can be compressably secured by a clamp as shown in FIG. 11.Prior to attaching the lid and clamping, the wells of the plate areloaded according to the solutions shown in the Table of FIG. 12. The lidis attached and the array assembly is clamped. The entire clampedassembly is placed on a rocker in an oven set to 70° C. A three-holemasking strategy is used so that times of 30, 60, and 120 minutes can betried in each well and for each solution. After all the experiments, aprofilometer is used to measure the etch depth for each solution. Asolution's reactivity to removing coating is determined from the depthof the etch pit.

Results are reported in the table shown as FIG. 12. The table shows thatthe surfactant offers no improvement. The acid mixtures of columns 7, 8and 10 are the most reactive as shown by the deepest etch pits. Incolumn 8, row 8, the 20/20 Phos/HFS solution shows the most rapidremoval of coating, however, at 120 minutes there is some base metalattack. The use of the Rodine inhibitor in the amount of 2-3% is foundto stop base metal attack at 120 minutes. Hence, the solutions of column10, rows 3 & 4 are selected as the best solutions.

While preferred embodiments of the invention have been described, thepresent invention is capable of variation and modification and thereforeshould not be limited to the precise details of the example. Forexample, the invention can be used to develop chemical etches for thesemiconductor industry, for functionalizing tail groups ofself-assembling monolayers (SAM's), for corrosion studies, fordevelopment of liquid based catalysts and in electrochemistry to (a)remove coatings, (b) deposit films and coatings, (c) electropolish asubstrate, (d) develop new electrolytes for batteries, fuel cells, etc.

In another embodiment, the method and assembly are used to select anumber of satisfactory solutions, which are then subjected to varyingtemperature, time periods, etc., to select optimize operationalconditions.

The invention includes changes and alterations that fall within thepurview of the following claims.

What is claimed is:
 1. A combinatorial high throughput screening wellarray assembly, comprising: (A) a metal substrate; (B) a mask placedupon said substrate to define an array of wells upon said substrate; and(C) a reservoir array plate atop said mask to provide a reservoir foreach of the wells to hold a candidate solution; wherein said maskdefines an array of wells each having a prescribed contact area withsaid substrate, and a ratio of reservoir volume capacity to a contactarea of wells of the array is determined by a molarity of a component ofsaid candidate solution, a composition and a thickness of a coating tobe stripped from said substrate; and wherein the substrate is a metaltest coupon base.
 2. The assembly of claim 1, wherein a volume capacityof said reservoir is determined by molarity change of said candidatesolution as a function of coating to be removed.
 3. The assembly ofclaim 1, wherein said mask defines an array of wells each having aprescribed contact area with said substrate and said mask is areplaceable mask that can be replaced by a different mask that defines adifferent contact area on said substrate from the contact area definedby said replaceable mask.
 4. The assembly of claim 1, wherein thesubstrate is a metal test coupon base from a turbine engine part.
 5. Theassembly of claim 1, wherein the substrate is a region of a turbineengine part.
 6. A combinatorial high throughput screening system,comprising said well array assembly of claim 1, and a reaction vessel toreceive said well array assembly.
 7. A combinatorial high throughputscreening well array assembly, comprising: (A) a metal test coupon base;(B) a mask placed upon said a metal test coupon base to define an arrayof wells upon said substrate; and (C) a reservoir array plate atop saidmask to provide a reservoir for each of the wells to hold a candidatesolution; wherein said mask defines an any of wells each having aprescribed contact area with said metal teat coupon base and said maskis a replaceable mask that can be replaced by a different mask thatdefines a different contact area on said substrate from the contact areadefined by said replaceable mask.
 8. The assembly of claim 7, whereinthe metal test coupon base is a region of a turbine engine part.
 9. Acombination high throughput screening system, comprising said well arrayassembly of claim 7, and a reaction vessel to receive said well arrayassembly.